Attention has been drawn to fuel cells using hydrogen and oxygen, as a power generating system which presents substantially no adverse effects on the global environment because in principle, its reaction product is water only. Among them, in recent years, a polymer electrolyte fuel cell using a proton conducting ion exchange membrane (polymer electrolyte membrane) as an electrolyte is expected to be useful for in-vehicle power sources, since its operation temperature is low, its output density is high and it can be downsized.
The polymer electrolyte fuel cell comprises a membrane/electrode assembly having electrodes (an anode (fuel electrode) and a cathode (air electrode)) disposed on both sides of a polymer electrolyte membrane; and separators having a gas channel formed on a surface thereof. Each electrode is normally composed of a catalyst layer in contact with the polymer electrolyte membrane and a gas diffusion layer disposed outside the catalyst layer. The gas diffusion layer plays a role in diffusing air or fuel in the electrode and a role in discharging water produced in the electrode.
The polymer electrolyte fuel cell is generally produced by disposing the membrane/electrode assembly between two separators to form a cell and stacking plural cells.
The polymer electrolyte fuel cell is characterized in that its operation temperature is low (from 50 to 120° C.) as mentioned above. On the other hand, however, it has a difficulty such that exhaust heat can hardly be utilized effectively for e.g. an auxiliary power. In order to offset such a difficulty, the polymer electrolyte fuel cell is required to have a high performance in utilization of hydrogen and oxygen, i.e. high energy efficiency and high output power density.
In order that the polymer electrolyte fuel cell satisfies the above requirement, the membrane/electrode assembly is particularly important among elements constituting the polymer electrolyte fuel cell.
Heretofore, the catalyst layer of the electrode is prepared from a viscous mixture obtained in such a manner that a catalyst powder for facilitating an electrode reaction and an ion exchange resin for increasing conductivity and preventing clogging (flooding) of a porous body due to condensation of water vapor are dissolved or dispersed in an alcohol solvent such as ethanol.
Processes for producing the membrane/electrode assembly are, for example, the following processes (1) to (3).
(1) A process comprising the steps of: forming catalyst layers by directly applying the above-mentioned viscous mixture onto surfaces of a polymer electrolyte membrane or, transferring or bonding catalyst layers obtained by applying the mixture onto separate sheet substrates, onto surfaces of the polymer electrolyte membrane, thereby to obtain a laminate of the catalyst layer/polymer electrolyte membrane/catalyst layer; and disposing a gas diffusion layer of a porous conductive material such as carbon paper or carbon cloth on each of two sides of the laminate.
(2) A process comprising the steps of: forming a laminate of a catalyst layer and a gas diffusion layer by directly applying the above-mentioned viscous mixture onto the gas diffusion layer to form the catalyst layer; and disposing the laminate on each of two sides of the polymer electrolyte membrane so that the catalyst layer is adjacent to the polymer electrolyte membrane.
(3) A process comprising the steps of: forming each catalyst layer by applying the above-mentioned viscous mixture onto a substrate; forming an electrode by laminating carbon paper or the like directly onto the catalyst layer by hot pressing; and bonding the electrode on each of two sides of the polymer electrolyte membrane by hot pressing or the like (for example, cf. Patent Document 1).    Patent Document 1: JP-A-2001-283864